Methane is abundantly available in nature in the form of natural gas, which typically contains about 75% methane by weight. Methane is also produced by other means, such as anaerobic digestion. Although methane is used primarily as a fuel, it is also a valuable starting material in the production of a number of important higher molecular weight saturated and unsaturated hydrocarbons, such as ethane, ethylene and acetylene. These compounds, in turn, are useful starting materials in the production of other commercially important petrochemicals.
Processes are known for converting methane to ethane, ethylene, acetylene, and hydrogen using a technique known as "pyrolysis." Such processes use heat to convert methane into higher molecular weight hydrocarbons without the presence of substantial amounts of oxygen gas or so-called free oxygen. A general discussion of the high temperature pyrolysis of methane can be found, for example, in chapter 1 of Pyrolysis: Theory and Industrial Practice (Academic Press, 1983), edited by L. Albright, B. Crynes, and W. Corcoran.
Methane can be converted into higher molecular weight hydrocarbons by a number of other processes which also involve the use of pyrolysis. For example, Gorin U.S. Pat. No. 2,488,083 describes a process for converting methane into normally liquid hydrocarbons by first converting methane into methyl halide, and then pyrolytically condensing the methyl halide into the desired end products. In this process, lower pyrolysis temperatures are made possible by the use of metal-based alumina-silica catalysts.
Sofranko, et al. U.S. Pat. No. 4,544,784 also describes a process for converting methane to higher hydrocarbons by pyrolysis. This process involves the use of heterogeneous reducible metal oxide catalysts with a halogen promoter.
Benson U.S. Pat. No. 4,199,533 describes a method for producing higher molecular weight hydrocarbons by igniting a mixture of methane and recyclable chlorine catalyst in a reaction chamber substantially devoid of free oxygen.
Methane can also be converted into benzene and other aromatic hydrocarbons by first converting it into methyl halide and then pyrolizing the resulting methyl halide. Such a process is described, for example, in Gorin U.S. Pat. No. 2,320,274.
A significant disadvantage to the use of the above processes for methane conversion is that in addition to producing the desired end products, these processes also produce substantial amounts of high surface area solid carbon or soot. Weissman and Benson have experimentally analyzed and verified the phenomenon of carbon soot formation in "Pyrolysis of Methyl Chloride, a Pathway in the Chlorine-Catalyzed Polymerization of Methane," International Journal of Chemical Kinetics, vol. 16, pp. 307-333 (1984). This solid carbon soot by-product, although potentially valuable, is difficult and costly to handle and dispose of. In addition, in substantial quantities, it is responsible for "poisoning," or destroying the effectiveness of, metal-based catalysts which may be used in the methane conversion process. Because of carbon soot formation, the overall cost of the above methods of methane conversion is increased.
Processes are also known for converting methane into so-called "synthesis gas," a mixture of carbon monoxide (CO) and hydrogen (H.sub.2). Synthesis gas is an important industrial feedstock, since it can be catalytically converted into methanol and several other useful chemicals in accordance with known processes. Current methods of producing synthesis gas typically involve the reformation of methane with steam at a high temperature (1000.degree. K) in the presence of a metal-based catalyst. One drawback of these processes is the exceedingly high temperatures required. Such high temperatures require a significant expenditure of energy and result in a corresponding increase in the cost of these processes.
None of the methane conversion processes described above are capable of simultaneously producing both higher molecular weight hydrocarbons and synthesis gas in an economically feasible and efficient manner. An efficient process for simultaneously producing higher molecular weight hydrocarbons and synthesis gas would be of great value to the petrochemical industry, since methane could be converted at the well site into feedstock for methanol and other important petrochemicals. The advantages of converting methane into such products at the well site are significant, because the resulting products are substantially less expensive and less dangerous to transport than methane gas.